Several methods of coating flowing liquids or a plurality of liquids are known in the art with the two most common types being curtain coating and slide-bead coating. Curtain coating has the disadvantage of requiring a free flowing curtain of solution which must be carefully controlled and monitored for uniformity across the substrate such as illustrated in U.S. Pat. No. 4,135,477 and references sited therein. Slide-bead coating is known in the art to provide a means of supplying a plurality of liquids to a coating efflux surface in which a bead is formed in a gap between a moving substrate and a lip of the coating efflux surface. The bead is normally maintained by differential pressure between the top of the flowing liquid and the bottom of the flowing liquid. The bead typically and preferable extends the width of the coating efflux surface (i.e. perpendicular to the direction of liquid flow) with the bead edge defined by an edge guide which is attached to the efflux surface. The edge guide acts as a physical barrier to control the width of liquid flowing onto the moving substrate.
The outer boundaries of the plurality of liquids are not controlled in the gap region between the coating lip on the coating efflux surface and the moving substrate. Variations in the bead tend to propagate at the bead edge thereby disrupting the integrity of the bead causing coating defects. An increase in differential pressure is typically required to maintain the integrity of the bead, however, higher differential pressure tends to cause other defects such as comb marks and streaks, for example. Improving the stability of the bead edge allows for the use of lower differential pressure thereby guarding against other defects while still maintaining a stable bead. The loss of bead integrity is a major cause for the loss of material which is unsuitable for the end use. This effect is more pronounced as the coating speed is increased.
The disclosed invention demonstrates a means for establishing and maintaining the bead edge by the use of an improved bead edge guide.